Neutron stars are the most compact concentrations of "normal" matter
known in nature, with the strongest gravitational fields. They are the
collapsed remnants of the burned-out cores of stars that started out
life with masses larger than about eight times that of our sun.
Everything about neutron stars is extreme: their density, gravity,
magnetic field and spin. They are spheres not larger than New York City,
which contain over 400,000 times the mass of Earth, so compressed that a
thimblefull of their material contains as much matter as half a million
Boeing 747 Jumbo jets filled with passengers and cargo. Their surface
gravity attraction is a hundred billion times that on Earth, their
magnetic fields are typically a trillion times stronger than that of
Earth, and their spin frequencies can be higher than 700 times per
second. In the past 44 years, over 1800 neutron stars have been
discovered, most of them as radio pulsars, regularly pulsing sources of
radio waves, but also hundreds as sources of X and . rays. Stellar black
holes are the burned-out cores of stars more massive than about twenty
times the sun. They can become X-ray sources if they are in binary
systems, stealing matter from a companion star.

Edward van den Heuvel
was born in 1940 in Soest, The
Netherlands and received his Ph.D. at the University of Utrecht in 1968.
He worked at the University of California, Santa Cruz from 1968 to 1969
and at the Universities of Utrecht, 1969 to 1974, and Brussels, 1970 to
1980. Since 1974 he has been Professor of Astrophysics at the University
of Amsterdam and, until 2005, Director of the Astronomical Institute
there. In 1995 he was awarded the Spinoza Prize, the highest science
prize of the Netherlands, and in 2002, the EU Descartes Prize, the
highest science prize of the European Commission, Brussels. Professor
van den Heuvel.s fields of expertise include stellar evolution, the
physics of neutron stars and black holes, X and .-ray astronomy and
radio pulsars.